21.13 @pagesizes [width][, height]: Custom Page Sizes

You can explicitly specify the height and (optionally) width of the main
text area on the page with the @pagesizes command. Write this
on a line by itself near the beginning of the Texinfo file, before the
title page. The height comes first, then the width if desired,
separated by a comma. Examples:

@pagesizes 200mm,150mm

and

@pagesizes 11.5in

This would be reasonable for printing on B5-size paper. To emphasize,
this command specifies the size of the text area, not the size of
the paper (which is 250mm by 177mm for B5, 14in by
8.5in for legal).

To make more elaborate changes, such as changing any of the page
margins, you must define a new command in texinfo.tex or
texinfo.cnf.